Discusses dimorphism of Oxalis; one form has 99% sterile anthers. Has found three kinds of fertile anthers.

Transcription

Desterro, Brazil

Octbr. 1st. 1866.

My dear Sir.

In your last letter (of May 23rd) you wished me to observe
whether Oxalis here exhibits different forms. I already wrote you
that in one of our species I found the relative lengths of the styli and stamens to be
variable; I have now satisfied myself that this species is dimorphic. In both the forms the stigmas reach to, or project a little beyond, the mouth of the
calyx and the longer stamens are fully one third longer than the styli. The shorter
stamens in one form are about half as long, whereas in the other they are about as long
as the styli.— Thus far the species offers no particular interest; but there
is one circumstance which renders it highly curious. At least 99 per cent of
the plants of the second form have perfectly sterile anthers, not producing a single
pollen-grain. Specimens of this form having all the anthers of all their flowers fertile
are extremely rare. There are three different estates of the anthers: with good
pollen—with small aborted pollen-grains—and without pollen; they
blend into one another by insensible steps; (thus you may find anthers with only two or
three good pollen-grains); but notwithstanding they are generally to be distinguished
even without the aid of the microscope, the fertile anthers being bright yellow, those
with aborted pollen-grains pale yellow and the pollen-less anthers white.

Now in nine stalks the flowers showed the following combinations of the different
anthers:

[DIAG HERE]

In the first form the anthers are always fertile; but even here, as in the fertile
anthers of the second form there is among the good pollen a large amount of aborted
pollen-grains.

In some rather rare specimens of the second form,—but only, as far as I know,
among those with sterile anthers,—the length of the stamens is very variable;
(such were casually the few plants, which I had examined, when writing my last
letter). Sometimes, in a single flower the longer stamens are
hardly as long, and the shorter ones half as long as the styli; but even in those
specimens the stamens and styli of the majority of flowers use to show the
characteristic lengths of the form.

It is not very rare to find one or two of the white pollen-less anthers transformed
into small leaflets, and I met with a single plant, (now in my garden), in which all the
anthers are transformed into petala-like leaves, sometimes exceeding ten in number; the
ovaries form five open leaves with green stigmas on the tip, but without ovula, and in
the midst of these ovarial leaves there is a great number of petala-like leaves, among
which sometimes some sepala may be distinguished.

Although the plant is extremely common (thousands flourishing in the very streets of
our town) I have not yet been able to find a single seed-capsule. This sterility may be
owing in part to the circumstance of the two forms but seldom growing mingled. You may find your way bordered, for a mile or more, by one of the two
forms exclusively, which then suddenly is substituted by the other.

A second very common species of Oxalis, with small yellow flowers, is
monomorphic, the styli reaching to the level of the longer stamens; this species
produces plenty of seed-capsules.

Of a third species with fine crimson flowers, ressembling in shape, size and colour
those of Linum grandiflorum, I have as yet seen only one plant, in which the styli were
intermediate in length between the two sets of stamens; although the stigmas of all the
flowers were densely covered with pollen, I found no seed-capsules; thus I am inclined
to think that it will prove dimorphic or trimorphic.

In short, I hope to have an opportunity of examining a fourth species of Oxalis with
woody stem.—

To the list of dimorphic plants I may add a Cordia, of which I enclose a
short- styled as well as a long-styled specimen; as soon as they are ripe, I shall send
you seeds of either form.—

In my last letter (of Aug. 2d) I alluded to the circumstance of
brightly coloured seeds remaining attached to the valves of the capsules after their
opening. I have since found two more plants, in which this is the
case. One is a twining Leguminosa, perhaps a Rhynchosia, with black and red seeds. The
other is a tree, probably belonging to the Mimoseae, which after the opening of the
seed-capsules presents a truly magnificent aspect, being covered over and over with
large and elegant curls of pale yellowish silk, (the spirally contracted valves) beset
with brilliant red pearls.

Octbr. 3. Yesterday I procured to calculate the number of seeds
contained in a large capsule of an Orchid (Maxillaria?); the seeds weighed 4212 grains; I extended half a grain of the seeds into a narrow heap and by
counting a small portion I found them to be 20661, so that the capsule would contain the
prodigious quantity of 1.756.440 seeds!

[DIAG HERE]

Hoping that this will find you in good health I am dear Sir with the most sincere
respect | Yours very truly | Fritz Müller

Do the aërial roots of all the species of Catasetum grow perpendicularly
upwards into the air, as they do in our Cat. mentosum?

See letter from Fritz Müller, 2 August 1866 and
n. 2. After further observations, Müller later came to regard this
species of Oxalis as trimorphic (see n. 6, below).

+

f3 5226.f3

In his work on dimorphic and trimorphic flowers, CD noted any differences in the
pollen of different forms of flowers (see `Dimorphic condition in Primula',
`Two forms in species of Linum', and `Three forms of Lythrum
salicaria'). CD had recently been interested in the observations made by his son
William Erasmus Darwin on the differences in the size of pollen-grains in Rhamnus
cathartica, but was unsure of the significance of such differences. William had
suggested that R. cathartica was an example of a dimorphic species becoming
dioecious (see letter to W. E. Darwin, 22 June [1866] and
n. 3). CD included Müller's description of this species of
Oxalis in Forms of flowers, pp. 180--1.

+

f4 5226.f4

Müller initially had noted considerable variation in the length of the
styles, even in flowers of the same plant (see letter from Fritz Müller,
2 August 1866).

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f5 5226.f5

In Forms of flowers, p. 180, CD mentioned that the stamens in some
of the flowers were `partially converted into petals'.

+

f6 5226.f6

Müller eventually reinterpreted the cause of sterility in this species of
Oxalis, as well as its dimorphic status, in light of CD's experimental
results with the illegitimate offspring of various heterostyled species. He concluded
that the plants might be the variable and sterile offspring of a single form of a
trimorphic species, accidentally introduced and afterwards propagated asexually (see
Forms of flowers, pp. 180--1; see also letter from Fritz
Müller, 31 October 1868, Calendar
no. 6439).

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f7 5226.f7

CD discussed this monomorphic species of Oxalis in Forms of
flowers, pp. 181--2.

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f8 5226.f8

In Forms of flowers, p. 180, CD referred to a trimorphic
Oxalis with red flowers that, according to Müller, had never
produced seed.

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f9 5226.f9

The specimens have not been found (see letter to Fritz Müller, [before
10 December 1866] and n. 5).

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f10 5226.f10

CD cut out the following paragraph and pasted it into his Experimental notebook
along with his own notes on brightly coloured seeds (DAR 157a: 80; the fragment has
since been moved to DAR 157a: 103). For Müller's earlier comments on the topic,
see the letter from Fritz Müller, 2 August 1866 and
nn. 14 and 15.

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f11 5226.f11

Müller evidently sent specimens of these seeds, but they have not been
found (see letter to J. D. Hooker, 5 December [1866] and
letter to Fritz Müller, [before 10 December 1866]).

+

f12 5226.f12

The sketch is reproduced at approximately 80 per cent of its original size.